Question

Draw skeletal structures of the following compounds (Sections \(2.6 \text { and } 2.7)\) (a) \(\mathrm{PhCH}(\mathrm{OH}) \mathrm{CH}_{2} \mathrm{CH}\left(\mathrm{NH}_{2}\right) \mathrm{CH}_{3}\) (b) \(\mathrm{EtCOCH}(\mathrm{Me}) \mathrm{CO}_{2} \mathrm{Me}\) (c) \(\mathrm{Me}_{2} \mathrm{C}=\mathrm{CHCH}_{2} \mathrm{CH}_{2} \mathrm{C}\left(\mathrm{CH}_{3}\right)=\mathrm{CHCH}_{2} \mathrm{OH}\) (d) 5 -(3-nitrophenyl)-5-oxopentanoic acid.

Short answer

Draw each compound by following the skeletal structure conventions and ensuring all functional groups and substituents are correctly placed.

Step-by-step solution

Understand Skeletal Structures

Skeletal structures, also known as line-bond structures, are simplified organic chemistry notations where lines represent carbon-carbon bonds, and carbon atoms are found at the endpoints and intersection of these lines. Hydrogens are assumed to be present where needed to satisfy carbon's four bonds, and functional groups are usually shown explicitly.

Drawing Compound (a)

For the compound \( \text{PhCH(OH)CH}_2\text{CH(NH}_2\text{)CH}_3 \), start by drawing a phenyl group, which is a hexagon for benzene. Attach a carbon to the phenyl group to represent \( \text{CH} \), which will then have a hydroxyl group (OH) attached. Move along the chain to add two more carbon atoms \( \text{CH}_2 \) followed by a carbon with an amino group \( \text{NH}_2 \) completing the chain with a methyl group \( \text{CH}_3 \).

Drawing Compound (b)

For \( \text{EtCOCH(Me)CO}_2\text{Me} \), begin by drawing an ethyl group \( \text{Et} \) which is a two-carbon chain. Attach a carbonyl group \( \text{CO} \) and continue by adding a carbon attached to a methyl \( \text{Me} \). Next, add another carbonyl \( \text{CO} \) and attach a methoxy group (from \( \text{OMe} \), where O is connected to a CH3).

Drawing Compound (c)

For \( \text{Me}_2\text{C=CHCH}_2\text{CH}_2\text{C(CH}_3\text{)=CHCH}_2\text{OH} \), start by drawing a central double bond between a carbon attached to two methyl groups \( \text{Me}_2\text{C=} \) and another carbonatic double bond with a hydrogen \( =\text{CH} \). Follow the chain by adding two \( \text{CH}_2 \) groups. Attach the terminal group with a carbon double-bonded \( \text{C(CH}_3\text{)=} \) leading to another double-bonded carbon \( \text{CH} \), finish the chain with the final \( \text{CH}_2 \text{OH} \) group, where OH is the alcohol group.

Drawing Compound (d)

For \( 5 \text{-(3-nitrophenyl)-5-oxopentanoic acid} \), start by understanding it has a pentanoic acid chain with a 3-nitrophenyl substituent at position 5 and a ketone (oxo group) also at position 5. Draw a 5-carbon chain with a carboxylic acid \( \text{COOH} \) at one end. At the fifth carbon, draw an additional bond to a 3-nitrophenyl group (a benzene ring with a nitro group at the third carbon) and to an oxo group \( \text{CO} \).

Verify and Adjust

After sketching each structure, verify that each carbon is tetravalent (four bonds), all substituents properly placed, and each functional group has the correct configuration.

Key concepts

Organic Chemistry Notation

Organic chemistry notation is a system of symbols and lines used to represent molecules in a simplified manner. This notation allows chemists to quickly convey complex molecular structures without cluttering the paper with all atoms. In skeletal structures, carbon atoms are implied at the vertices and endpoints of lines. One line represents a single bond, double lines represent double bonds, and triple lines represent triple bonds.
Hydrogen atoms are usually not explicitly shown unless they are part of a functional group, as their placement is easily inferred due to carbon's stable tetravalency (four bonds per carbon).
Most importantly, functional groups that define the chemical behavior of the molecule, such as alcohols ( (OH) ), amines ( (NH_2) ), and carboxylic acids ( (COOH) ), are drawn explicitly because of their significant importance. This streamlined notation is particularly helpful when analyzing large organic molecules or complex chemical reactions.

Drawing Chemical Structures

Drawing chemical structures involves translating a chemical formula into a visual representation using organic chemistry notation. In this process, the skeletal form provides an easy-to-draw representation of molecular topology and spatial configuration. For example, while drawing compound (a) from the exercise, we start with phenyl (Ph) represented as a hexagonal benzene ring. Lines are drawn outwards for carbon chains or branches.
By indicating functional groups, such as hydroxyl (OH) and amino (NH_2), the student demonstrates molecular connectivity and highlights reactive sites. Drawing these structures becomes intuitive with practice, as often the overall arrangement follows logical chemsitry principles, like attaching functional groups to relevant carbon atoms in a systematic chain or branch.
While initially daunting, practicing these drawings assists in visualizing organic compounds more efficiently, solidifying understanding of molecular geometry and bonding, which are cornerstones of organic chemistry.

Functional Groups Representation

Functional groups are specific groupings of atoms within molecules that are responsible for the characteristic chemical reactions of those molecules. Their representation in organic molecules is essential as they influence the properties and reactivity of the compound.
In skeletal structures, functional groups are the only parts of a molecule that are typically shown in detail, ensuring clarity about chemical functionality. Common functional groups include hydroxyl (OH) for alcohols, amine ( (NH_2) ) for amino acids, and carboxyl ( (COOH) ) for carboxylic acids.
These groups are central to molecule behavior. For instance, in compound (d), the carboxylic acid group (COOH) denotes acidity and reactivity, while a nitro group ( (NO_2) ) modifies electronic properties and introduces unique reactivity pathways. In chemical diagrams, showing these groups explicitly allows students to predict and rationalize chemical reactivity, synthesis pathways, and interaction with other molecules or environments. Understanding and recognizing these groups is crucial for mastering organic chemistry.